1. Field of the Invention
The present invention relates to the field of cables for transmitting and/or distributing electricity. In particular, the present invention relates to a method for making a splice between optical fibres in a device for forming a joint between electrical cables, particularly (but not exclusively) medium- or high-voltage cables.
2. Description of the Related Art
A cable for transmitting and/or distributing electricity typically comprises one or more conductive cores. Each conductive core generally comprises a metal conductor surrounded by an insulating layer and preferably by an inner semiconducting layer and an outer semiconducting layer. In particular, if the cable is a high-voltage type, the conductive core or cores are in turn surrounded by a metal screen (generally made of aluminium, lead or copper), outside which a protective polymer sheath is provided.
In the present description, the term “medium voltage” refers to a voltage typically in the range from about 1 KV to about 30 KV, while the term “high voltage” refers to a voltage of more than 30 KV.
In order to join two electrical cables (each having a single conductive core, for example), a portion of each of their ends is stripped, so that the metal conductors are exposed. The metal conductors of the two cables are then connected electrically and a joint device is fitted on their ends. The primary function of the joint device is to insulate the conductors electrically and to provide mechanical protection for the area of the joint between the cables.
A joint device generally comprises a sleeve which is fitted directly on the ends of the cables, and an outer protective casing in which the sleeve is housed. A sleeve fitted directly on the ends of the cables is described for example in patent application EP 0735639 A1. An outer protective casing is described for example in patent application EP 2113978 A1.
The sleeve may comprise different layers, each having a different function. For example, starting from the innermost layer, a sleeve may comprise a tubular electrode made of a semiconducting material which performs the function of controlling the electrical field, a tubular insulator which covers the electrode, and an outer semiconducting covering which covers the tubular insulator and which performs the function of interconnecting the ends of the outer semiconducting layers of the two joined cables. The sleeve usually has a substantially cylindrical central portion and two substantially conical opposing end portions, through which the two cable ends enter the sleeve.
After the sleeve has been fitted on the joined cable ends, it is housed in the outer casing, which is typically made of a metallic material (such as copper), as described in the above mentioned patent application EP 2113978 A1, and which performs the function of ensuring electrical continuity between the metal screens of the two joined cables.
An electrical cable for transmitting and/or distributing energy can also comprise one or more optical fibres. The optical fibres can be used, for example, for transmitting data, or for monitoring the operation of the electrical cable. The optical fibres of an electrical cable are usually housed in a protective metal tube, which is in turn inserted among the metal wires of the metal screen of the cable.
When two ends of electrical cables comprising optical fibres are joined together, the optical fibres must also be spliced together.
The document “Raychem Fibre Optic Add-On Kit for HV Cable Accessories”, which can be found at the URL:
http://energy.tycoelectronics.com/transmission/fibreoptic.pdf, describes a kit for installing optical fibres in a joint device for high-voltage electrical cables. The kit comprises a splice tray which is housed in a suitable compartment of the outer casing. The compartment and the splice tray are accessible from outside the casing.
The kit described above has a number of drawbacks.
First of all, the need to provide a splice tray and a corresponding compartment for housing it disadvantageously entails an increase in the cost and overall dimensions of the outer casing, and therefore of the joint device as a whole. Furthermore, since the splice tray is positioned so as to be accessible from the outside of the casing, additional mechanical protection must be provided in order to protect adequately the optical fibres housed therein, and this further increases the cost and overall dimensions of the outer casing and therefore of the joint device as a whole. On the other hand, the accessibility of the splice tray from the outside of the outer casing is generally superfluous, since—once the splices have been made between the optical fibers—it is not normally necessary to carry out any other operation on the optical fibres.